1. Field of the Invention
The present invention relates to a method and a plant for rolling hot-rolled wide strip from continuously cast thin slabs of ferritic/pearlitic microalloyed structural steels with a microalloy with vanadium and/or with niobium and/or with titanium in a CSP plant or compact strip production plant, wherein the cast slab strand is supplied divided into rolling lengths through an equalizing furnace to a multiple-stand CSP rolling train and is continuously rolled in the CSP rolling train into hot-rolled wide strip, is then cooled in a cooling stretch and is reeled into coils.
2. Description of the Related Art
EP-A-0368048 discloses the rolling of hot-rolled wide strip in a CSP plant, wherein continuously cast initial material, after being divided into rolling lengths, is conveyed through an equalizing furnace directly to the rolling mill. Used as the rolling mill is a multiple-stand mill in which the rolled lengths which have been raised to a temperature of 1100.degree. C. to 1130.degree. C. in the equalizing furnace are finish-rolled in successive work steps, wherein descaling is carried out between the work steps.
In order to achieve an improvement of the strength and the toughness properties and the corresponding substantial increase of the yield strength and the notch value of a rolled product of steel, EP-A-0413163 proposes to thermomechanically treat the rolling stock.
In contrast to normalizing deformation in which the final deformation takes place in the range of the normal annealing temperature with complete recrystallization of the austenite, in the case of the thermomechanical deformation temperature ranges are maintained for a specified deformation rate in which the austenite does not recrystallize or does not significantly recrystallize, i.e., prior to the actual thermomechanical treatment of the rolling stock, an austentite structure is always present which does not contain any nuclei or structure components or only very small portions thereof in the phase which is stable at low temperature.
The adjustment of this initial structure can be effected either directly from the casting heat or in a preheating furnace from room temperature or an intermediate temperature.
In the method proposed in EP-A-0413163, the transformation of the rolling stock begins in the temperature range of the stable austenite and continues to just above the A.sub.r3 temperature. In order to reach the most favorable temperature range for thermomechanical rolling, the initial pass temperature is determined in dependence on the desired degree of deformation.
A significant feature of the thermomechanical treatment is the utilization of the plastic deformation not only for manufacturing a defined product geometry, but also especially for adjusting a desired real structure and, thus, for ensuring defined material properties, wherein non-recrystallized austenite is subjected to the polymorphous gamma (.gamma.)--alpha (.alpha.)--deformation (in the normalizing deformation the austentite is already recrystallized).
Prior to deformation in a conventional rolling mill, conventional slabs when used in the cold state are subjected to the polymorphous transformations:
melt (L).fwdarw.ferrite (.delta.).fwdarw.austentite A.sub.1 (.gamma.).fwdarw. PA1 ferrite (.alpha.).fwdarw.austentite A.sub.2 (.gamma.) PA1 melt (L).fwdarw.ferrite (.delta.).fwdarw.austentite A.sub.1 (.gamma.) PA1 with an increased oversaturation of the mixed crystal austentite and an increased precipitation potential for carbonitrides from the austentite. PA1 a) changing the casting structure by adjusting defined temperature and shape changing conditions during the first deformation, wherein the temperature is above the recrystallization stop temperature T.sub.R, so that during and/or after the first deformation a complete dynamic and/or meta-dynamic and/or static recrystallization of the casting structure takes place prior to the beginning of the second deformation step; PA1 b) deforming in the last roll stands at temperatures below T.sub.R temperature, wherein the deformation should not fall below an amount of 30% and the final rolling temperature is near the A.sub.r3 temperature (temperature of the austentite/ferrite transformation); PA1 c) controlled cooling of the hot-rolled wide strip in the cooling stretch, preferably a laminar cooling stretch, wherein the polymorphous transformation of the austentite takes place at a temperature which is between the A.sub.r3 temperature and the B.sub.s temperature (bainite starting temperature). PA1 in a conventional rolling mill, a slab with recrystallized structure which has been rough-rolled in the roughing train (plastically deformed) enters the finishing train; PA1 in the CSP finishing train, the thin slab enters with cast structure; PA1 the surface properties of a CSP thin slab differ substantially from a rough-rolled slab, for example, with respect to its topology. PA1 by a different mobility of the high-angle grain boundaries; PA1 different mixed crystal and precipitation behavior; PA1 different diffusion mechanism and kinetics due to the different character of the boundary surfaces and the chemical inhomogeneities which also must be taken into consideration when adjusting the method parameters.
while the following is true for the CSP technology: